Returns from land, sea and weather are regarded as clutter in an air search radar or in other radars. These returns can be suppressed in a signal processor of the radar system when the spectrum is narrow compared with the radar's pulse repetition rate (prf). Filters that combine two or more returns from a single range cell are able to discriminate between the desired targets and clutter. This allows the radar to detect targets with cross-sections smaller than that of the clutter. It also provides a system of preventing the clutter from causing false alarms.
Two classes of clutter filters are moving target indicator (MTI) and pulse Doppler. MTI combines a few return pulses, usually two or three, in a way that causes the clutter returns to cancel. A pulse Doppler process is class of clutter filter where the returns in each range resolution cell are gated and put into a bank of Doppler filters. The number of filters in the bank approximately equals the number of pulse returns combined. Each filter is tuned to a different frequency and the pass bands contiguously positioned between zero frequency and prf. The pulse Doppler technique is most often used in either airborne or land based target tracking radars, where a high ambiguous prf can be used, thus providing an unambiguous range of Doppler frequencies. The filter bank may be instrumented digitally by a special purpose computer formatted according to a Fast Fourier Transform (FFT) algorithm, for example.
Moving target indicator (MTI) filtering is used in many radar systems with the main purpose to reduce the amount of low or zero Doppler clutter in order to enhance the detectability of targets which may be masked by the clutter. Limitations to the degree of clutter rejection can often be limited by aspects of the system design which impact on pulse-to-pulse stability. Often, severe clutter rejection requirements can impose very strict pulse-to-pulse stability requirements on the radar systems analog electronics which can considerably increase its cost.
The present solution measures the pulse-to-pulse systematic variations of a radar system and removes them from the processing. In this way, the system pulse-to-pulse stability requirements are relaxed, which will result in less expensive hardware while improving the clutter rejection capability of the radar system.